As regards the current thread concerning magnetic field reversals, may I
recommend the following:
"Geodynamo Turns Toward a Stable Magnetic Field," Physics Today, January
1996, pp. 17-18
*** This brief article describes the work of Glatzmeier and Roberts in
simulating the Earth's magnetic field numerically. Their model
spontaneously underwent a field reversal.
"In Earth's Magnetic Pas de Deux, the Inner Core Leads," Physics Today,
September 1996, pp. 17-19
"Earth's Solid Core Outruns its Crust," Sky & Telescope, October 1996, p. 10
*** These brief articles describe the evidence that the inner core of the
Earth rotates more rapidly than the crust.
E. Nesme-Ribes, S. L. Balunias, D. Sokoloff, "The Stellar Dynamo,"
Scientific American, August 1996, pp. 47-52
*** This article describes reversals in the solar magnetic field.
More about magnetic field reversals can be found in any good geology text.
I recommend F. Press and R. Siever, UNDERSTANDING EARTH (W. H. Freeman,
1998).
In addition, a particularly nice web site for geology is